The formation of micromechanical structures using photolithography and sacrificial layers is known in the art. The present invention is an improvement to the technology disclosed in U.S. Pat. No. 5,190,637 to Gluckel, titled “Formation of Microstructures by Multiple Level Deep X-ray Lithography with Sacrificial Metal Layers,”(“the '637 patent”), which is incorporated herein by reference. In the '637 patent, microstructures are built in several steps. First, a layer of photoresist capable of exposure by radiation is applied to a plating base. The photoresist is then exposed to radiation using a mask. The mask allows the radiation to only “expose” a certain defined area of the photoresist. Subsequent developing removes photoresist selective to the exposure creating a cavity that replicates the plan of the exposure mask. The cavity formed in the photoresist is then filled by a primary metal that is electroplated on to an exposed plating base. The remaining photoresist is then removed and a secondary metal (a sacrificial metal) is electroplated over the entirety of the primary metal and plating base. The primary metal and the secondary metal are then machined by mechanical means down to a height which exposes the primary metal and planarizes the surface for subsequent processing. After machining, another layer of photoresist may be applied across both the primary and secondary metals, and then this photoresist is also patterned using the same procedures as above. After the primary metal has been electroplated into the cavity created in the photoresist, the rest of the photoresist is removed and the secondary metal is electroplated over the entirety of the first secondary metal, any exposed first primary metal and the newly added second primary metal. Both the second primary and second secondary metals are machined down to the desired thickness of the second layer of the primary metal and the process is repeated until the desired number of layers have been formed creating the desired microstructure in the primary metal. Once the microstructure has been formed, the entirety of the plating surface together with the primary and secondary metals are exposed to an etching agent that selectively etches away the secondary metal but not the primary metal, thus leaving only the primary metal and the plating surface.
The secondary metal is used instead of only using the photoresist because of the structural stability that it affords the primary metal during machining. Machining cannot generally be done through both the photoresist and the primary metal because the photoresist is relatively weak mechanically and may not adequately support the primary metal from damage due to the largely lateral forces encountered in the mechanical machining process (which may include machining, grinding, lapping, polishing, chemo-mechanical polishing, electric discharge machining, or any other commonly encountered machining process. Likewise, the photoresist may not be removed first before the machining of the primary metal because of the likelihood of substantial damage to the primary metal structure, such as the tearing or ripping off of portions of the structure that are not laterally supported. Finally, an added advantage the secondary metal affords is that it conveniently provides a conductive plating base for subsequent layers of primary metal that overhang underlying primary metal structures. Otherwise, the overhanging plating would require an additional thin film seed layer deposition step.
However, significant problems arise using this method when multiple or very laterally large microstructures are built on a single substrate, such as when manufacturing semiconductor testing probe heads. Plating the secondary metal across the entirety of the plating surface, for instance ceramic in the case of the probe heads, causes the plating base to bow and warp under the stress of the additional sacrificial metal. This causes two related problems: 1) it becomes difficult or impossible to machine the different layers to a uniform thickness and 2) it becomes difficult or impossible to perform the lithography because micro-lithography requires a planar surface.
Several different techniques have been used or suggested to address this problem, however, each has its deficiencies. Firstly, the use of secondary (sacrificial) metal with a controlled low stress has been attempted. Plated deposit stress can be controlled through the use of additives or through pulse-plating techniques. However, during subsequent processing, such as photoresist curing, the applied heat may cause the secondary metal to become “stressy” (meaning to apply film stress relative to the substrate) thus the warping is not prevented. Attempts have also been made to increase the thickness of the substrate, knowing that the stress which the substrate can withstand increases with the thickness. However, the thickness of the substrate necessary to withstand the stress of the secondary metal may be impractical. Additionally, the film stress (which is most often, though not necessarily, tensile in nature) causes mechanical failure at or near the substrate/metal interface, including both delamination and bulk fracture of the ceramic.
Thus, what is needed is a process for creating micromechanical and micro electro-mechanical system structures, such that multiple or large structures may be built on the same substrate, without the substrate warping.